The OTRAG rocket is an interesting design. Vehicles would be assembled from 4 to hundreds of identical rocket modules. By using an extremely simple design, producing the modules cheaply using economies of scale, and not bothering with recovery and refurbishing, it was expected to be 1/10 the cost of similar vehicles. (Basically, the exact opposite of the shuttle's philosophy.)
The Wikipedia article is a fascinating read of what can be done with a simple, clever design.
It appears that the program failed entirely due to geopolitical reasons. OTRAG was a German company, and France and the Soviet Union did not want Germany to have another rocket program. The launches occurred in Congo and Libya, both run by dictators. The owner of OTRAG did not want to give 50% control to Americans, which would be legally required for U.S. assistance. Nonetheless, there were 18 test flights of the concept. The linked page claims that it was human-rated (if that can be believed).
Have there been any other launch vehicle designs like the OTRAG? Specifically, the vehicle should be (1) built of modular units and (2) non-reusable. So for example, the Ares family would qualify for the former but not the latter.